As a constant voltage circuit that steps down a power supply voltage (an input voltage) to be supplied and generates a constant voltage to output it to a load coupled to its output terminal, a linear regulator circuit such as an LDO (Low Drop Out) circuit exists. FIG. 12 is a circuit diagram depicting a configuration example of a conventional linear regulator circuit. In FIG. 12, 101 denotes an error amplifier circuit, 102 denotes an output transistor using a P-channel transistor, and 103 denotes a load coupled to an output terminal of the linear regulator circuit. Further, Vref denotes a constant reference voltage supplied from a not-illustrated reference voltage circuit and Vout denotes an output voltage output from the output terminal of the linear regulator circuit.
In the linear regulator circuit depicted in FIG. 12, when the output voltage Vout becomes lower than the reference voltage Vref, an output voltage of the error amplifier circuit 101, namely a voltage to be supplied to a gate of the output transistor 102 drops. As a result, an on resistance of the output transistor 102 reduces and the output voltage Vout rises. Conversely, when the output voltage Vout becomes higher than the reference voltage Vref, the output voltage of the error amplifier circuit 101 rises. As a result, the on resistance of the output transistor 102 increases and the output voltage Vout drops. In this manner, the output voltage Vout output from the output terminal of the linear regulator circuit is maintained to the reference voltage Vref being a constant voltage.
Here, in the linear regulator circuit depicted in FIG. 12, when in the case of a current flowing through the load being quite small, a leakage current flowing between a drain and a source of the output transistor 102 increases such as at the time of high temperature, the output voltage Vout becomes uncontrollable to rise up to a power supply voltage level. This is because even though the output transistor 102 operates in an off direction according to the output of the error amplifier circuit 101, the output voltage Vout continues to rise due to the leakage current generated in the output transistor 102.
In order to avoid the above problem, there has been proposed a method in which a high voltage is supplied to a back gate (that is also referred to as a substrate gate) of an output transistor to thereby increase a threshold voltage of the output transistor and reduce a leakage current of the output transistor (see, for example, Patent Documents 1 and 3). In the Patent Document 1, in order to suppress the leakage current of the output transistor, there has been disclosed a configuration in which according to an operation mode, a voltage to be supplied to a back gate of an output transistor is switched by a switch. In the Patent Document 1, at the time of a standby mode when a load is brought into a low current consumption state rather than a normal operation, for suppressing the leakage current, it is controlled that a voltage HVcc higher than a voltage LVcc at the time of the normal operation is input to the back gate of the output transistor. Further, in the Patent Document 3, there has been disclosed a configuration in which a voltage to be supplied to a back gate of an output transistor is switched from VDD1 to VDD2 (>VDD1) when it becomes a certain temperature or higher, and thereby a leakage current of the output transistor under a high temperature condition is suppressed. Further, there has been proposed a technique in which according to a drive current value of an output transistor, a voltage to be supplied to a back gate of the output transistor is switched by a switch (see, for example, Patent Document 2).
[Patent Document 1] Japanese Laid-open Patent Publication No. 2007-206948
[Patent Document 2] Japanese Laid-open Patent Publication No. 2002-116829
[Patent Document 3] Japanese Laid-open Patent Publication No. 2004-94788
In the above-described conventional constant voltage circuit, the voltage to be supplied to the back gate of the output transistor is switched by a switch, but there is a problem that in the switching operation, a rapid fluctuation in the output voltage due to switching noise is caused. The above fluctuation in the output voltage causes malfunctions of a logic circuit and an analog circuit caused by power noise, element breakdown by occurrence of a pulse voltage equal to or higher than a withstand voltage, and the like.